1. Field of the Invention
This invention relates to microwave frequency signal attenuators. In particular, it relates to a microwave frequency attenuator compatible with microstrip type circuit construction.
Desirable characteristics of microwave frequency attenuators are a high power handling capability and a flat frequency response over as wide a range of frequencies as is practical. Microwave frequency devices are very sensitive to structures which affect the field relations in the electrodes. As a consequence, attenuation characteristics may vary with frequency unless careful attention is given to the structural characteristics of devices intended for achieving the ideal attenuation characteristic.
The structure of high frequency circuits is particularly critical in integrated or near integrated (hybrid) circuit construction. Conventional integrated circuit construction is predisposed to design of circuits in a flat essentially single plane with minimal attention to field effects. Unfortunately, at microwave frequencies, the fields cannot be readily retained within the structurally desirable single plane along the path of propagation. As a consequence, parasitic fields may be generated in such structures which admit to signal interference and signal loss, as fields may interfere or otherwise attenuate a signal. What is therefore needed is a high frequency, i.e., microwave, signal attenuation device capable of flat frequency response from essentially zero frequency to a signal range where the wavelength is comparable to the size of the circuit, have high power handling capability and have a structure which is easily employed at essentially any attenuation level without any special design considerations.
2. Description of the Prior Art
U.S. Pat. No. 3,260,971 to the present inventor and E. R. Seitter issued July 12, 1966 describes a multilayered card attenuator for microwave frequencies which employed a distributed attenuator in a coaxial configuration. While suitable for coaxial configurations, the structure is unsuited to microstrip circuit applications.
U.S. Pat. No. 3,157,846 issued to B. O. Weinschel for a card attenuator for microwave frequencies describes another distributed coaxial microwave attenuator. The Weinschel patent describes a device which is essentially limited to single value resistive layers. This type of device has been found to exhibit disadvantages of poor flatness for attenuation of low value, e.g., in the 1 to 6 db range, and leakage problems at higher attenuation values (50 db to 100 db) especially at the higher frequencies, e.g., above 15 GHz.
U.S. Pat. No. 3,824,506 issued to the present inventor for microwave attenuators describes a still further coaxial distributed-resistance attenuator. In this patent, the distributed resistance is formed in the shape of a hollow tube or hemitube or hemicylinder in which the field lines are generally perpendicular to the resistive film.
U.S. Pat. No. 4,309,677 to Goldman describes a microstrip tee attenuator network in which attenuation elements are employed in a discrete structure. One of the embodiments described is a plated through circular hole. The Goldman patent teaches that the plated through hole minimizes undesired parasitic impedances normally encountered in prior art attenuator networks. While Goldman recognizes the need to minimize undesired parasitic impedances, the structure fails to satisfactorily minimize those undesired parasitic impedances.
U.S. Pat. No. 4,310,812 issued to DeBloois for a high power attenuator and termination having a plurality of cascaded tee sections is illustrative of the single plane construction of prior art microstrip structures. The patent describes an attenuator having discrete shunt elements connected to a ground wrapped around the substrate so as to provide a connection in a single plane along the surface of the substrate. The electric fields are perpendicular to the shunt path giving rise to undesired parasitic impedances.